In the search for high performance materials, considerable interest has been focused upon carbon fibers. Such carbon fibers contain at least 90 percent carbon by weight and commonly are formed by the thermal treatment of a polymeric fibrous precursor. The term "carbon fibers" is used herein in its generic sense and includes graphite fibers as well as amorphous carbon fibers. Graphite fibers are defined herein as fibers which have a predominant X-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers which exhibit an essentially amorphous X-ray diffraction pattern. Graphite fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.
Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus. Graphite is one of the very few known materials which tensile strength increases with temperature. Uses for carbon fiber reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels, electrical heaters, hot rolls, bearings, low-shrinkage molds, and ablative materials for heat shields on re-entry vehicles.
In the prior art numerous materials have been proposed for use as possible matrices in which carbon fibers may be incorporated to provide reinforcement and produce a composite article. The matrix material which is selected is commonly a thermosetting resinous material and is commonly selected because of its ability to also withstand highly elevated temperatures. Metallic matrix materials may also be utilized.
Heretofore carbon fibers commonly have been subjected to some form of surface modification treatment in order to enhance their ability to adhere to a matrix material. For instance, various techniques have been proposed in the past for modifying the fiber surface properties of a previously formed carbon fiber in order to make possible improved adhesion when present in a composite article. See, for instance, U.S. Pat. No. 3,476,703 and British Patent No. 1,180,441 to Nicholas J. Wadsworth and William Watt wherein it is taught to heat a carbon fiber normally within the range of 350.degree. C. to 850.degree. C. (e.g. 500.degree. to 600.degree. C.) in an oxidizing atmosphere such as air for an appreciable period of time. Other atmospheres contemplated for use in the process include an oxygen rich atmosphere, pure oxygen, or an atmosphere containing an oxide of nitrogen from which free oxygen becomes available such as nitrous oxide and nitrogen dioxide. Carbon fiber surface modification processes involving treatment in a gaseous atmosphere are disclosed in commonly assigned U.S. Pat. Nos. 3,723,150; 3,723,607; 3,745,104 and 3,754,957. Other carbon fiber surface treatments involving the use of acids are referred to in Belgian Patent No. 708,651, British Patent No. 1,238,308 and U.S. Pat. Nos. 3,597,301, and 3,660,140. Such surface treatments result in an insignificant pick-up of bound oxygen upon the fiber surface, e.g. less than about 0.05 percent by weight, and commonly no substantial reduction in the bulk electrical conductivity of the fibrous material, e.g. less than about a 0.1 percent reduction. These surface treatments have been concerned with achieving an interaction between surface carbon atoms and the reacting medium to form suitable complexes capable of promoting adhesion between the fiber and a resin matrix during composite article formation. The bulk properties such as density, elastic moduli, conductivity, and internal microstructure, are unaffected by such prior art surface treatments.
It is an object of the present invention to provide an improved specifically defined process for the internal chemical modification of a carbonaceous fibrous material.
It is an object of the present invention to provide an improved process for modifying the bulk physical properties of carbon fibers.
It is an object of the present invention to provide an improved process for substantially lowering the electrical conductivity of carbon fibers.
It is an object of the present invention to provide an improved process for substantially lowering the thermal conductivity of a carbonaceous fibrous material.
It is an object of the present invention to provide an improved process for modifying the bulk physical properties of a carbonaceous fibrous material without significant deterioration of other important fiber properties, e.g. tensile strength, strain to failure, and corrosion resistance.
It is an object of the present invention to provide novel carbon fibers exhibiting an electrical conductivity which differs from the ordinary electrical conductivities of about 450 to 1,600 ohm.sup.-1 cm.sup.-1 commonly exhibited by carbon fibers at 25.degree. C.
It is an object of the present invention to provide novel carbon fibers containing about 3 to 30 percent bound oxygen by weight, having an average single filament tenacity of at least 200,000 psi, and exhibiting a substantially reduced electrical conductivity, e.g. about 0.1 to 300 ohm.sup.-1 cm.sup.-1 measured at 25.degree. C.
It is another object of the present invention to provide chemically modified carbon fibers having a substantially lower elastic modulus, and a substantially higher strain to fracture.
These and other objects, as well as the scope, nature, and utilization of the invention, will be apparent to those skilled in the art from the following detailed description and appended claims.